36      PROCESS  FOR  OBTAINING  AND  PURIFYING  GLYCERINE. 
M.  Cap's  process,  though  an  undoubted  improvement,  was  not 
perfect,  as  glycerine  so  purified  is  always  liable  to  contain  more 
or  less  of  salts  of  lime  ;  and  some  glycerine,  purified  in  our 
laboratory  according  to  M.  Cap's  directions,  contained  in  addition 
volatile  fat  acids  ;  and  though  the  process  was  known  in  this 
country,  specimens  of  the  so-called  "  pure  "  glycerine  obtained 
from  the  best  sources  in  London,  so  recently  as  last  January, 
contained  in  every  case  more  or  less  impurity. 
The  best  specimen  came  from  Edinburgh ;  but  even  this  was 
not  absolutely  free  from  impurity.  Some  medical  men  appear 
to  have  been  afraid  to  prescribe  glycerine  for  internal  use,  some- 
times with  reason  as  appears  from  the  Chemist  of  May,  1855, 
whe^e  Mr.  Hamilton,  of  Liverpool,  referring  to  the  papers  of 
MM.  Cap  and  Garot,  and  of  Dr.  Crawcour,  stated  that  no  doubt 
the  glycerine  purified  and  used  by  them  might  be  safely  used  in- 
ternally ;  but  that  having  doubts  about  the  purity  of  the  gly- 
cerine commonly  sold  as  "  pure  glycerine,"  he  had  procured 
samples  from  several  of  the  most  respectable  Chemists  in  Liver- 
pool, and  on  examination  had  detected  lead  in  considerable 
quantity,  and  that  the  specimen  in  which  he  detected  the  largest 
quantity  of  lead  was  labelled  "  pure  glycerine,"  was  sold  at 
double  the  price  of  the  common  glycerine,  and  was  warranted 
free  from  lead. 
I  will  now  proceed  to  describe  the  new  process  for  obtaining 
and  purifying  glycerine,  and  may  remark,  that  the  road  by 
which  we  arrived  at  pure  glycerine,  was  rather  a  circuitous  one. 
Our  first  step  was  to  do  away  with  the  lime  process  of  saponifica- 
tion, and  with  it  our  only  source  of  impure  glycerine.  By  our 
first  improvement  in  separating  the  fat  acids  from  neutral  fats, 
the  glycerine  was  decomposed  by  the  direct  action  of  concentra- 
ted sulphuric  acid  at  a  high  temperature,  and  all  that  remained 
of  it  was  a  charred  precipitate.  A  new  process  for  decomposing 
neutral  fats  by  water  under  great  pressure  coming  under  our 
notice,  led  us  to  look  again  more  closely  into  our  old  distilling 
processes ;  and  the  doing  this  showed,  what  we  had  often  been 
on  the  brink  of  discovering,  that  glycerine  might  be  distilled. 
In  our  new  process,  the  only  chemical  agents  employed  for  de- 
composing the  neucral  fat  and  separating  its  glycerine,  are  steam 
And  heat ;  and  the  only  agents  used  in  purifying  the  glycerine 
